Investment casting is a well-known technique for producing articles having, among other features, internal cavities. The cavities may be necessary for weight reduction, containment capacity or flow-through capability. The investment casting process has been found to be very useful for fabrication of complex metal castings, especially those having hollow internal cavities. By properly supporting patterns made of an easily removable substance, such as wax, very complex internal configurations can be produced.
Gas turbine engines utilize hollow components, primarily for weight reduction and for cooling capability. Cooling is achieved by flowing bleed air through some of the components, particularly airfoils such as blades and vanes in the turbine section, where the highest operating temperatures are encountered, and where the efficiency of the engine is most limited by the capability of the materials to withstand the effects of high temperatures. By appropriate cooling, the operating temperatures can be raised to levels which would otherwise destroy, or severely shorten the lifetime of, uncooled components. A typical air cooled vane is shown in FIG. 1.
In addition to flowthrough cooling, air is frequently bled from the internal cavity through the airfoil walls so that it flows over the outer surface of the airfoil to provide film cooling. Common methods of forming the air outlets through the airfoil walls include electron beam and laser drilling, and electrical discharge machining (EDM). While these techniques have been successfully employed for many years, the cooling passages are essentially restricted to a line-of-sight configuration. They also require extra manufacturing steps, involving time-consuming and labor intensive processes, and are thus very expensive.
Techniques have been developed whereby ceramic mini cores are embedded in the wax patterns so that, when the wax patterns are removed after formation of the ceramic mold around the wax pattern, the mini cores remain as part of the mold and define the pathways through the airfoil component by which the cooling air flows from the inner cavity of the airfoil to its outer surfaces.
To achieve the proper cooling of the airfoil without diverting excessive incoming air, which would adversely affect the efficiency of engine operation, very close tolerance metering holes are required to control the amount of air flowing through the cooling passages.
Initial attempts at casting the metering holes, using extensions of the mini core which form the metering holes, hereinafter called metering hole pins, resulted in excessive breakage of the metering hole pins, which seemed to occur during the wax removal portion of the mold fabrication process. These difficulties encountered with casting in of the metering holes initially dictated that the metering holes be formed after the casting process, generally by an EDM technique. Again the extra manufacturing steps required are time consuming and expensive.
Thus it is necessary to have a method of casting in the metering holes so that a simple machining operation opens the holes to air flow. It is further necessary to have a method which permits formation of the investment casting mold without damaging the metering hole pins.